deep compare CTEs before considering them conflicting

deep compare CTEs before considering them conflicting

Fixed issue where referencing a CTE multiple times in conjunction with a
polymorphic SELECT could result in multiple "clones" of the same CTE being
constructed, which would then trigger these two CTEs as duplicates. To
resolve, the two CTEs are deep-compared when this occurs to ensure that
they are equivalent, then are treated as equivalent.

Fixes: #8357
Change-Id: I1f634a9cf7a6c4256912aac1a00506aecea3b0e2
(cherry picked from commit 85fa363c846f4ed287565c43c32e2cca29470e25)